Synthesis and characterization of a modified "picket fence" porphyrin complex - stronger π bonding interactions between Fe(ii) and axial ligands

Perspectives on Ligand Properties of N-Heterocyclic Carbenes in Iron Porphyrin Complexes

There has been considerable research interest in the ligand nature of N-heterocyclic carbenes (NHCs). In this work, two six-coordinate NHC iron porphyrin complexes [Fe^II(TTP)(1,3-Me₂Imd)₂] (TTP = tetratolylporphyrin, 1,3-Me₂Imd = 1,3-dimethylimidazol-2-ylidene) and [Fe^III(TDCPP)(1,3-Me₂Imd)₂]ClO₄ (TDCPP = 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin) are reported. Single-crystal X-ray characterizations demonstrate that both complexes have strongly ruffled conformations and relatively perpendicular ligand orientations which are forced by the sterically bulky 1,3-Me₂Imd NHC ligands. Multitemperature (4.2-300 K) and high magnetic field (0-9 T) Mössbauer and low-temperature (4.0 K) EPR spectroscopies definitely confirmed the low-spin states of [Fe^II(TTP)(1,3-Me₂Imd)₂] (S = 0) and [Fe^III(TDCPP)(1,3-Me₂Imd)₂]ClO₄ (S = 1/2). The similarity of 1,3-Me₂Imd and imidazole, as well as the well-established correlations between the ligand nature and spectroscopic characteristics of [Fe^II,III(Porph)(L)₂]^0,+ (Porph: porphyrin; L: planar base ligand) species, allowed direct comparisons between the pair of ligands which revealed for the first time that NHC has a stronger π-acceptor ability than imidazoles, in addition to its very strong σ-donation.

Bis(1-methylimidazole)[meso-α,α,α,α-tetrakis(o-nicotinamidophenyl)porphinato]iron(II)–1-methylimidazole–tetrahydrofuran (1/1/1.5)

The crystal structure of the bis­(1-methyl­imidazole)-ligated iron(II) picket-fence porphyrin derivative [Fe^II(C₆₈H₄₄N₁₂O₄)(C₄H₆N₂)₂]·C₄H₆N₂·1.5C₄H₈O is investigated.

In the title compound, [Fe^II(C₆₈H₄₄N₁₂O₄)(C₄H₆N₂)₂]·C₄H₆N₂·1.5C₄H₈O, the central Fe^II ion is coordinated by four pyrrole N atoms of the porphyrin core and two N atoms of the 1-methyl­imidazole ligands in the axial sites. One 1-methyl­imidazole and one and a half tetra­hydro­furan solvent mol­ecules are also present in the asymmetric unit. The complex exhibits a near planar porphyrin core conformation, in which the iron centre is slightly displaced towards the hindered porphyrin side (0.01 Å). The average Fe—N_p (N_p refers to the pyrrole nitro­gen atoms in the porphyrin) bond length is 1.990 (9) Å, and the axial Fe—N_Im (N_Im refers to the imidazole nitro­gen atoms) bond lengths are 1.993 (3) and 2.004 (3) Å. The dihedral angle between the two coordinated 1-methyl­imidazole planes is 56.6 (2)°. The dihedral angles between the 1-methyl­imidazole planes and the planes of the closest Fe—N_p vector are 16.8 (2) and 39.8 (2)°. N—H⋯N and N—H⋯O inter­actions are observed in the crystal structure.

Intermolecular Interactions and Intramolecular Couplings of Binuclear Porphyrin Models for Cytochrome c Oxidase

Cytochrome c oxidase (CcO) has a binuclear active site composed of a high-spin heme group and a tris-histidine-ligated copper ion (Cu_B). By using two different porphyrin models derived by Gunter (H₂TPyPP) and us (H₂TImPP), we have isolated several mono- and binuclear complexes including one carbonyl and three chloride derivatives which are determined by 100 K single-crystal X-ray. Low-temperature (4 K) EPR and multitemperature (295-25 K) Mössbauer investigations on the products not only confirmed the spin states of the two metal ions (S = 5/2 Fe³⁺ and S = 1/2 Cu²⁺) but also revealed the intermolecular interactions and intramolecular couplings which are in accordance with the crystal structural features.

Synthesis and characterization of six-coordinate iron(II/III) 5,10,15,20-tetrakis(pentafluorophenyl) porphyrinato complexes with non-hindered imidazole ligands

Four bis-imidazole iron(II/III) 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato (TFPP) complexes, [Fe(TFPP)(1-MeIm)[Formula: see text]], [Fe(TFPP)(1-VinylIm)[Formula: see text]], [Fe(TFPP)(4-MeHIm)[Formula: see text]]Cl and [Fe(TFPP)(1-EtIm)[Formula: see text]]BF[Formula: see text] (1-MeIm [Formula: see text] 1-methylimidazole, 1-VinylIm [Formula: see text] 1-vinylimidazole, 4-MeHIm [Formula: see text] 4-methylimidazole and 1-EtIm [Formula: see text] 1-ethylimidazole) were synthesized and characterized by single-crystal X-ray and UV-vis spectroscopy. A negative correlation is found between the absolute imidazole orientation ([Formula: see text] and the Fe–N[Formula: see text] distance for the [Fe(II)(Porph)(Im)[Formula: see text]] (Im [Formula: see text] 1-MeIm or 4-MeHIm) complexes where the smaller [Formula: see text] angle corresponds to a longer axial distance. Hydrogen bonding, which might affect the orientations of the axial imidazoles is found for Fe(TFPP)(4-MeHIm)[Formula: see text]]Cl (A and B). The autoreduction of [Fe(III)(TFPP)]Cl to [Fe(II)(TFPP)(1-MeIm)[Formula: see text]] with 1-methylimidazole has been monitored by UV-vis spectroscopic titration.

Histidine-Lysine Axial Ligand Switching in a Hemoglobin: A Role for Heme Propionates

The hemoglobin of Synechococcus sp. PCC 7002, GlbN, is a monomeric group I truncated protein (TrHb1) that coordinates the heme iron with two histidine ligands at neutral pH. One of these is the distal histidine (His46), a residue that can be displaced by dioxygen and other small molecules. Here, we show with mutagenesis, electronic absorption spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy that at high pH and exclusively in the ferrous state, Lys42 competes with His46 for the iron coordination site. When b heme is originally present, the population of the lysine-bound species remains too small for detailed characterization; however, the population can be increased significantly by using dimethyl-esterified heme. Electronic absorption and NMR spectroscopies showed that the reversible ligand switching process occurs with an apparent pK_a of 9.3 and a Lys-ligated population of ∼60% at the basic pH limit in the modified holoprotein. The switching rate, which is slow on the chemical shift time scale, was estimated to be 20-30 s^-1 by NMR exchange spectroscopy. Lys42-His46 competition and attendant conformational rearrangement appeared to be related to weakened bis-histidine ligation and enhanced backbone dynamics in the ferrous protein. The pH- and redox-dependent ligand exchange process observed in GlbN illustrates the structural plasticity allowed by the TrHb1 fold and demonstrates the importance of electrostatic interactions at the heme periphery for achieving axial ligand selection. An analogy is drawn to the alkaline transition of cytochrome c, in which Lys-Met competition is detected at alkaline pH, but, in contrast to GlbN, in the ferric state only.

Porphyrinoid rotaxanes: building a mechanical picket fence

Building on recent progress in the synthesis of functional porphyrins for a range of applications using the Cu-mediated azide-alkyne cycloaddition (CuAAC) reaction, we describe the active template CuAAC synthesis of interlocked triazole functionalised porphyrinoids in excellent yield. By synthesising interlocked analogues of previously studied porphyrin-corrole conjugates, we demonstrate that this approach gives access to rotaxanes in which the detailed electronic properties of the axle component are unchanged but whose steric properties are transformed by the mechanical "picket fence" provided by the threaded rings. Our results suggest that interlocked functionalised porphyrins, readily available using the AT-CuAAC approach, are sterically hindered scaffolds for the development of new catalysts and materials.